The present invention relates to an improved structure of an antenna, in particular, relates to a room antenna having high enough gain and being small enough in size to be suitable for the reception of VHF and/or UHF band. The present antenna is in particular utilized for the reception of an FM broadcasting in 76-90 MHz in Japan.
One of the prior antennas of this kind is shown in FIG. 1A, which is called a ferrite antenna, having a vertical ferrite bar 1. The reference numeral 2 is a tank circuit having a coil 3 wound around the bar 1 and the variable capacitor 4, and 5 is an output coil wound around the ferrite bar 1. A ferrite antenna has been widely used since it provides high output voltage due to the high permiability of a ferrite bar. In FIG. 1A, the variable capacitor 4 provides the resonant condition of the tank circuit 2 by adjusting the capacitor 4, so that the resonant frequency of the tank circuit becomes equal to the desired reception frequency. And, the antenna output is provided from the output coil 5 through the output terminal 6.
The gain G of the ferrite antenna of FIG. 1A is shown below: EQU G=1.5.mu..sub.e Q.sub.e (3.pi..sup.2 /4) (S.multidot.L.sub.f (.lambda..sub.O)
where .mu..sub.e is the effective permiability of the ferrite bar, Q.sub.e is the effective Q of the ferrite bar, S is the cross sectional area of the ferrite bar, L.sub.f is the length of the ferrite bar, and .lambda..sub.O is the free space wavelength. Therefore, the gain G of the antenna is proportional to the values .mu..sub.e, Q.sub.e, S and L.sub.f.
FIG. 1B shows the relationships between the gain G of the ferrite antenna, and the length L.sub.f and the diameter .phi. of the ferrite antenna of FIG. 1A, where the values .mu..sub.e and Q.sub.e are given. In FIG. 1B, the vertical axis shows the gain G, the horizontal axis shows the length L.sub.f, and .phi. is the parameter. As apparent from FIG. 1B, the higher the gain G is, the larger the diameter .phi. and the longer the length L.sub.f are.
However, when the size of a ferrite bar exceeds a predetermined threshold value, the gain G of a ferrite antenna is decreased because of the loss by the ferrite core, and further, due to the self inductance and the stray capacitance of the antenna itself, the antenna resonates itself in VHF band. Further, since a ferrite antenna is non-directional in the horizontal plane, when the electromagnetic wave is reflected by buildings and/or other reflectors, the antenna receives an echo due to the multipath by the reflection.
Another prior antenna for VHF and/or UHF band is a half wavelength dipole antenna. However, a conventional dipole antenna has the disadvantage that the size is large as the length of a dipole antenna must be 1/2 of the wavelength.